Protective systems are designed to respond to faults such as short circuits in order to protect faulty equipment and associated circuits, power supplies and human operators from the consequences of current overload. There are many ways of de-energizing faulty equipment depending on factors such as zones of protection, relay speed, and reliability. Circuit breakers and fuses are commonly used to block a load current after it exceeds a predetermined magnitude known as break current. Fuses must be replaced when blown which implies a long duration outage. Circuit breakers are reset rather than replaced.
Mechanical circuit breakers often employ contacts that are pulled apart under excessive current and subsequently closed by pushing a reset button. Mechanical circuit breakers include thermal circuit breakers in which a thermal metal strip bends under high current to open the contacts. Mechanical circuit breakers also include magnetic circuit breakers which develop a sufficient magnetic field in a coil of wire under high current to activate a spring trigger mechanism to pull the contacts apart.
Solid state circuit breakers, that is, circuit breakers using solid state components, have been developed to provide advantages over mechanical circuit breakers. These advantages include faster response, smaller size, more accurate thresholds, quieter operation, and higher reliability. See, for instance, U.S. Pat. Nos. 5,221,847 and 3,668,483, "IC Functions As Programmable Electronic Circuit Breaker", Electronic Products, Jul. 1994, p. 79, and "Build Solid-State Circuit Breaker", Electronic Design, Jul. 22, 1993, p. 88.
Circuit breakers may be reset either automatically or by external means. Automatically resetting circuit breakers normally constrain the power source, for instance by requiring a minimum load current for load current to flow or requiring zero load voltage for reset to occur. In the event a circuit breaker is reset externally, whether by a computer or a human operator, it is essential that adequate electrical isolation be provided between the control circuit (input circuit) and the power source (output circuit). This concern is particularly acute with solid state circuit breakers. In solid state components, voltage isolation is often provided by gate oxide layers in metal-oxide-semiconductor (MOS) devices. Gate oxide layers are normally relatively thin to allow low turn-on threshold values. As a result, typical voltage isolation for gate oxide layers in on the order of 80-100 volts. Even gate oxide layers specially designed for high voltage isolation are usually incapable of withstanding more than a few hundred volts. A power surge that causes a high voltage to pierce a gate oxide layer and energize the control circuit could be catastrophic. Various solutions have been proposed. For example, U.S. Pat. No. 3,668,483 employs a circuit breaker with an isolated control source, but the circuit is limited to dc load currents, uses a mechanical relay, and requires power from the load to operate a switch.
Thus, there is a need for a solid state circuit breaker which retains the aforementioned advantages over mechanical circuit breakers while providing high voltage isolation between the control circuit and the power source, bi-directional load currents, and few constraints on the power source.